Implement static file serving for plugin.

Now Response accepts a File object to serve the file as a static file.
File system module's `read` method will return a File object and we can
pass the object to the Response constructor.
Implement standalone mode for test.

There is no need to launch zookeepr for test. And restarting zookeeper
server for every test makes test slow.

This patch implements the option to launch standalone mode. The
standalone mode will not make routing table so it doesn't need to launch
zookeeper.